A conventional projection exposure apparatus projects a circuit pattern of a reticle (mask) onto a wafer via a projection optical system in manufacturing a semiconductor device using the photolithography technology. A practice of a projection exposure apparatus that uses the EUV light having a wavelength between about 5 nm and 20 nm (“EUV exposure apparatus” hereinafter) smaller than a wavelength of the ultraviolet light is studied so as to meet the demand for the fine processing of a semiconductor device.
In order to precisely transfer a reticle pattern onto a wafer at a predetermined magnification, the projection optical system needs a high imaging characteristic with few aberrations. However, along with the recent fine processing to the semiconductor device, the projection exposure apparatus cannot sometimes previously transfer the reticle pattern onto the wafer under the influence of the aberration of the projection optical system. Therefore, there is a demand for a precise measurement of the wavefront aberration of the projection optical system.
A lateral shearing interferometer (“LSI”) is one known apparatus that can precisely measure the wavefront aberration of the projection optical system applied to the EUV light, without the precise alignment that is required for a point diffraction interferometer. The LSI generally arranges a pinhole mask having one pinhole on an object plane of a target optical system. A pinhole image images on an image plane under the aberrational influence of the target optical system. A diffraction grating is arranged between the image plane and the target optical system to shear the wavefront in two orthogonal directions. As a result, an interference pattern is obtained on an observation plane subsequent to the image plane. When the wavefront information obtained from wavefront data in each direction is integrated, a two-dimensional wavefront is restored.
In order to take from a pinhole the light enough intensified for the wavefront measurement, the light from a high luminance light source should be condensed onto the pinhole. An undulator light source inserted into the electron storage ring is a conceivable high luminance light source, but requires bulk facility and increases the cost. In the assembly process and the installation at the designation of the exposure apparatus, the light source for the wavefront measurement is preferably small and common to the exposure light source.
On the other hand, the light from an exposure light source, such as a laser produced plasma light source (“LPP”) and a discharge produced plasma light source (“DPP”), has low directivity, and a difficulty in condensing onto the pinhole. Therefore, use of the exposure light source for the wavefront measurement result in a very small amount of the light that transmits the pinhole, and cannot provide the interference image necessary to measure the wavefront on the observation plane.
One proposed solution for the improvement of the light use efficiency to arrange a one-dimensional reflection-type grating on the object plane of the target optical system. See Japanese Patent Application, Publication No. (“JP”) 2005-079592. This reflection-type has a structure (reflection area) with a random height. Instead of arranging one pinhole on the object plane, the reflection area is wide enough to improve the light use efficiency. Moreover, JP 2005-079592 arranges a diffraction grating at an image plane position of the target optical system. In the wavefront measurement, the reflection-type grating is moved in parallel to the lines in order to restrain the speckles generated from the reflection-type grating, and then a phase of the diffraction grating is shifted to calculate a shearing wavefront or difference wavefront among the diffracted beams so as to calculate the wavefront of the target optical system.
However, the LSI using the phase shift generally needs to photograph plural interference images while shifting a phase difference among the diffracted beams of respective orders by a specific amount, and while changing shift directions. Thus, JP 2005-079592 needs to photograph two pairs of interference images, and requires a long time for measurements. Moreover, unless an optical element, such as a diffraction grating, is extremely stably held during shifting (or unless it is moved without changing the height direction), a measurement error occurs.